Neuromuscular electrical stimulation (NMES) (also referred to as powered muscle stimulation, functional muscle stimulation, electrical muscle stimulation, and other terms) is an established technology capable of activating a person's muscles involuntarily and non-invasively. NMES is typically delivered as an intermittent and repeating series of short electrical pulses. A complicating factor is that each person responds differently to NMES. Thus, it is often required to adjust stimulation parameters on a case-by-case basis to ensure that a person receives effective therapy that is both safe and well-tolerated. During adjustment for traditional NMES, a trained operator must be present to guarantee that stimulation parameters remain within a safe range of values. Even with a trained operator, parameter adjustment to achieve optimal results is typically an iterative and time-consuming process.
Zanotti and colleagues (Chest 124:292-296, 2003), incorporated herein by reference, have demonstrated improved functional outcomes and accelerated patient rehabilitation by applying NMES to the leg muscles of bed-bound COPD patients. Despite this and other clinical evidence showing improved patient outcomes, NMES technology has not been transferred for use in the intensive care unit (ICU) setting (where critically ill patients are cared for), although it has been hypothesized that doing so could improve patient care (Morris et al., Critical Care Clinics, 23:1-20, 2007—incorporated herein by reference). In its current state, NMES is inadequate for use in the ICU setting.
Two major factors provide a barrier to the use of NMES in the ICU: 1) the need for user training for safe and effective delivery of therapy and 2) the labor-intensive nature of current NMES devices. Most FDA-approved electrical muscle stimulators are designed for use in more than one application (ex. pain management, sports rehabilitation, muscle atrophy), and therefore leave many stimulation settings described above under the control of the operator (ex. a nurse, physical therapist). Virtually all nurses, as well as most physical and occupational therapists, are not trained to deliver NMES therapy and therefore do not have the knowledge base required to adjust stimulation parameters safely and effectively or to tailor energy levels on a patient-by-patient basis. A second muscle stimulation task requiring training involves placement of stimulation electrodes over the motor points of muscles. With traditional NMES, precise electrode placement is required if muscles are to be activated effectively in a manner such that the person receiving therapy experiences minimal discomfort. Current methods to determine electrode placement involve initial estimations based upon anatomical markers, followed by iterative trial-and-error based adjustments based upon an observed muscle response. Again, most nurses and physical therapists are not trained to perform these adjustments.
The second barrier to the use of NMES in the ICU is the lack of available personnel to deliver therapy. Even if current electrical stimulation devices were straightforward to use, stimulation electrode re-positioning and stimulation parameter adjustment is a labor-intensive activity. A recent study (Lacey et al., North Carolina Center for Nursing, 2002—http://www.nursenc.org/research/chgs_time_alloctn.pdf), incorporated herein by reference, found that time for direct patient care by nurses declined by 6% during the period of 1999-2001. Given skyrocketing health-care costs, many institutions cannot afford or cannot justify hiring additional help, especially well-compensated advanced operators trained in delivering NMES therapy. In particular, critical care nurses have their time fully committed, and cannot take on a new patient care activity without discarding another. Because NMES is not vital to a critically ill person's immediate survival, it's delivery would need to be very time-efficient in order for it to be implemented in the ICU setting. Existing electrical muscle stimulation devices found in the prior art do not meet this standard.
Within the ICU, the patient cohort comprised of sedated, comatose, or otherwise non-interactive patients poses unique challenges that further render existing electrical stimulation devices and treatment paradigms ineffective. Because patients are non-interactive, direct patient assessment of muscle contraction strength (often used to aid judgments of stimulation effectiveness) is unavailable. This leaves the onus of judgment to a device operator who is most often left with only visual evidence of contraction (i.e., looking for muscle and/or body part movement in treated regions). A striking example of the effect of this limitation arises if the target muscle group for stimulation is the quadriceps. As the overwhelming majority of sedated or comatose ICU patients lie in bed with legs extended, little to no physical movement is activated by stimulating quadriceps muscles, even though the process of stimulation is effectively preserving muscle mass and strength. Particularly in older patients with low baseline muscle mass, induced muscle contractions may be very difficult to distinguish visually. These difficulties exacerbate problems related to a modality that is already riddled with shortcomings.
Furthermore, although generally considered safe, NMES therapy is occasionally associated with skin and/or tissue burns. There are multiple potential causes of burns. One common cause is an excessive amount of current flowing through a small area of tissue (i.e., large current density). While the risk of this type of burn can be minimized through the use of large dispersive electrodes and mechanisms to ensure good electrode contact with a person's skin, burns of this type continue to occur. Another type of burn is associated with abnormally large temperature increases in the electrode itself, oftentimes due to an electrode malfunction. In this scenario, increases in electrode temperature may result in superficial skin burns, or more serious burns if the situation is not addressed. Over time, temperatures at the skin surface may also increase when using normally functioning electrodes simply due to resistive heating in skin, although in this scenario temperatures rarely reach dangerous levels. Stecker and colleagues (Am J END Tech., 43:315-342, 2006), incorporated herein by reference, provide an extensive review of the potential mechanisms of injury when using electrical skin electrodes.
Temperature control requirements during NMES seek to constrain temperatures within a range that is safe to avoid tissue burns. As noted by Prausnitz (Advanced Drug Delivery Reviews 18:395-425,2006), incorporated herein by reference, the required temperature rise for tissue damage is a function of the duration which the temperature rise is applied to tissue. For surface electrodes, temperature rises are generally desired to be less than 6° C. during NMES therapy. Given that average baseline skin surface temperatures generally do not exceed 33° C., it is desirable that temperatures above 39° C. should be avoided.
For most users, the risk of serious skin or tissue burn due to an abnormally hot electrode or a severe temperature rise at the skin interface is minimal. This is because the electrode can be removed or the NMES system disabled by the user before temperature rises become significant. For example, most persons receiving NMES would detect a painful or unpleasantly hot temperature shortly after an electrode malfunction (as the electrode begins to warm) and would be able to terminate therapy (or inform a trained operator that something is wrong) before temperatures continued to rise to more serious levels. In this scenario, minor skin irritation or skin burns could occur, but more serious skin or deep tissue burns are avoided.
Immobilized persons, however, are at increased risk for serious skin and deep tissue burns. A large proportion of immobilized persons are medical patients who are suffering from conditions such as coma or who are receiving interventions (such as mechanical ventilation) that generally require sedation and/or analgesia. These patients are likely to have abnormal skin sensation and/or a reduced sensory threshold. As a result, these patients have a reduced capacity to acknowledge that an electrode or region of skin is increasing in temperature. Thus, the risk mitigation mechanisms described above that exist for most users are not available to these persons. Accordingly, the U.S. FDA places a labeling requirement on marketing literature for powered muscle stimulators, indicating that caution must be utilized when electrodes are placed over skin areas lacking normal sensation.
The potential for severe burns is one of the major reasons that NMES therapy is not typically delivered to comatose, sedated, or analgesed patients in the intensive care units (ICUs) of most hospitals. Recent peer-reviewed medical literature has confirmed the potential benefits of NMES for immobilized ICU patients. However, in these very ill patients, the consequences of a serious burn (and subsequent risk of further infection, etc) are potentially devastating. Given that the focus of ICU care is to maintain life and stabilize a patient's vital signs, the risk of harmful burns outweighs any downstream benefits related to the maintenance of muscle strength. In the high demand environment of the ICU, a nurse or other care provider does not have the time or resources to constantly check stimulation electrodes to ensure proper functioning and a safe range of operating temperatures. Existing electrical stimulation devices do not provide adequate protection against burns when used with this vulnerable group of persons.
Therefore, a need exists for improved NMES systems and methods, which may be delivered to comatose, sedated, or analgesed subjects.